1. Field of the Invention
The present invention relates in general to electro-optic probe for sampling oscilloscopes for observing waveforms according to polarization states of target signals produced by coupling electric fields formed by the target signals to an electro-optic crystal and injecting optical timing pulses into the electro-optic crystal, and relates in particular to an electro-optic probe having an improved optical system.
This application is based on a patent application No. Hei 10-294566 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
It is possible to observe waveforms of target signals, by coupling electric fields formed by the target signals to an opto-electronic crystal, and injecting laser light into the crystal and observing the polarization state of the laser light. If the laser light is pulsed, the modulated target signals can be analyzed with a fine resolution. An optical sampling oscilloscope uses an electro-optic probe based on this phenomenon.
Electro-optic sampling oscilloscope (shortened to EOS oscilloscope) has received much attention because of the following special features of the instrument, compared to an oscilloscope using a normal electric probe:
(1) signal measurement is facilitated because the scope does not require a ground line;
(2) there is virtually no effect on the behavior of the target signals because the metal pin used at the tip of the electro-optic probe is electrically isolated from the circuit system to provide a high input impedance: and
(3) bandwidth of the measurable range is increased to a GHz range because optical pulses are used to modulate the target signals.
FIG. 5 shows a construction of a conventional electro-optic probe system comprised by: an electro-optic probe head 1 made of an electrical insulator having a metal pin 1a inserted in the center thereof; an electro-optic (e-o) crystal 2 having a reflection film 2a at the reflection-end, which is in contact with the metal pin 1a; collimating lenses 3, 8; a quarter-wave plate 4; polarizing beam splitters 5, 7; a Faraday element 6 for rotating the polarized plane of the injected light 45 degrees; a laser diode 9 for generating modulating laser light in response to modulating signals output from a pulsing circuit (not shown) provided in the main body 19 of the EOS oscilloscope; collimating lenses 10, 11; photo-diodes 12, 13 for converting the injected modulating laser light to electrical signals and outputting the electrical signals to the main body 19 of the EOS oscilloscope; an isolator device 14 comprised by the quarter-wave plate 4, the polarizing beam splitters 5, 7 and the Faraday element 6; and a probe casing 15 made of an electrical insulator.
Next, the optical path of the laser light generated from the laser diode 9 will be explained with reference to FIG. 5. In FIG. 5, incident laser beam is indicated by a letter A.
First, laser light emitted from the laser diode 9 is converted to a parallel beam of light by the collimating lens 8, and propagates in a straight line through the polarizing beam splitter 7, the Faraday element 6, and the polarizing beam splitter 5, and into the quarter-wave plate 4, and is focused by the collimating lens 3 to enter the e-o element 2. Incident light is reflected by the reflection film 2a formed on the surface at the reflection-end of the e-o element 2.
The reflected light is again converted to a parallel beam of light by the collimating lens 3, which passes through the quarter-wave plate 4, and a portion of the reflected beam is reflected at the polarizing beam splitter 5 and enters photo-diode 12, while the reflected light transmitting through the polarizing beam splitter 5 is reflected at the polarizing beam splitter 7 and enters photo-diode 13.
The quarter-wave plate 4 is for equalizing the intensities of the laser beams entering the photo-diodes 10,11.
The operation the electro-optic probe shown in FIG. 5 to measure target signals will be explained in the following.
When the metal pin 1a is made to contact a measuring point, Pockeles"" effect is generated by the voltage applied to the metal pin 1a, thereby altering the birefringence of the e-o element 2 due to piezo-electric effect. This causes changes in the polarization states of the incident laser light emitted from the laser diode 9 and propagating through the e-o element 2. The incident beam, with altered polarization states, is reflected by the reflection film 2a, and the signal beams produced at the beam splitters 5, 7 are converted to electrical signals in the photo-diodes 12, 13.
As the voltage of the measuring point changes, the changes are manifested in the polarization states, represented by the differences in the output signals from the photo-diodes 12, 13, which are caused by the electrical signals being sensed by the metal pin 1a. 
In the operation of the electro-optic probe explained above, the electrical signals obtained from the photo-diodes 12, 13 are input in the conventional EOS oscilloscope for processing; however, in place of this process, it is possible to measure target signals by using a conventional real-time measuring oscilloscope, by connecting it to the photo-diodes 12, 13 through dedicated controllers. This process permits measurements over a broad range of bandwidths using the conventional electro-optic probe.
However, in the design of the conventional electro-optic probe, because the optical axes of the photo-diodes 10,11 for receiving the signal beams generated at the reflection film 2a are placed transversely to the longitudinal optical axis of the injected laser light emitted from the laser diode 9, the terminals for the photo-diodes 10,11 are disposed in the radial direction of the probe casing 15. Furthermore, because the measuring cables are needed to be attached to the terminals, there is a need to provide transverse spaces to accommodate the cables, resulting in an excessive effective diameter of the probe casing 15. Because the probe is hand held by an operator to touch a measuring point such as a wiring junction in printed circuit board, such a bulky probe presents a serious impediment to delicate handling required in performing waveform measurements accurately.
It is an object of the present invention to provide an electro-optic probe having a slender probe casing to enable to carry out delicate measuring operations by reducing the diameter of the probe casing.
The object has been achieved in an electro-optic probe for an oscilloscope comprising:
a laser diode for emitting a modulating laser light according to control signals generated in a main body of the sampling oscilloscope;
a first lens for converting the modulating laser light to a parallel beam;
a second lens for focusing the parallel beam;
an opto-electronic element having a reflection film at a reflection-end;
an isolator device disposed between the first lens and the second lens for transmitting the modulating laser light and separating a reflected beam produced at the reflection film into signal beams;
and photo-diodes for converting optical energies of the signal beams separated by the isolator device into respective electrical signals; wherein,
the signal beams to enter the photo-diodes are directed to propagate towards the laser diode, and the photo-diodes are disposed in a longitudinal direction of a probe casing.
Additionally, because the present design of the probe places the photo-diodes laterally with respect to the laser diode within a common plane, so as to enable to align the optical axis of the signal beams, entering the photo-diodes, parallel with the optical axis of the modulating laser light. emitted from the laser diode, by bending the signal beams ninety degrees at the polarizing beam splitters. This design enables the probe diameter to be minimized by eliminating the need for providing transverse spaces for the components, thereby producing a slender probe suitable for performing delicate waveform measurements accurately.
It should be noted that the reference numerals appended to particular components in the claims do not limit the technical interpretation of the claims.